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Fruit split and fruit size studies on Citrus

Stander, Ockert Petrus Jacobus (2013-03)

Thesis (MScAgric)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Fruit size and the integrity of the rind are key components that determine the value of a citrus
fruit. The application of 2,4-dichlorophenoxy acetic acid (2,4-D) to reduce splitting, a
physiological disorder which entails cracking of the rind as well as to increase fruit size was
conducted on three different split-susceptible mandarin and two split-susceptible orange
cultivars. Treatments were applied directly after the physiological fruit drop period, as well as
in January and February at 10 mg·L-1, alone or in combination with calcium (Ca), potassium
(K) or gibberellic acid (GA3). Application of 2,4-D directly after physiological fruit drop,
either alone or in a tank-mix with K, consistently reduced the number of split mandarin fruit,
with later applications in January and February generally being ineffective. Post
physiological fruit drop application of 10 mg·L-1 2,4-D significantly increased growth rate
(mm.day-1) of all the mandarin cultivars, resulting in increased fruit size. Differences in
sensitivity of cultivars to 2,4-D were evident, with the January application reducing the
splitting in ‘Midknight’ Valencia. However, all the 2,4-D treatments reduced the fruit growth
rate of the orange cultivars. The 2,4-D treatments, in terms of splitting, increased rind
thickness, -strength and -coarseness of ‘Marisol’ Clementine, throughout fruit development.
In addition fruit diameter and –length increased to such an extent that the fruit shape was
altered (reduced d/l-ratio), reducing the potential of the rind to crack and the fruit to split,
however rind coarseness of treated fruit was also increased. There were no major negative
side effects on internal and external fruit quality, except for a possible reduction in juice
content (%). Therefore, 10 mg·L-1 2,4-D can be applied directly after physiological fruit drop
on ‘Marisol’ Clementine and ‘Mor’ mandarin to reduce fruit splitting.